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Monday, May 25, 2009

Thematic Maps

Five thematic mapping techniques that are used most often:

choropleth

proportional or graduated symbols

isarithmic or contour map

dot map

dasymetric

From about.com:

Types of Thematic Maps

Although cartographers can use these datasets in many different ways to create thematic maps, there are five thematic mapping techniques that are used most often. The first and most commonly used of these is the choropleth map. This is a map that portrays quantitative data as a color and can show density, percent, average value or quantity of an event within a geographic area. Sequential colors on these maps represent increasing or decreasing positive or negative data values. Normally, each color also represents a range of values.

Proportional or graduated symbols are the next type of map and represent data associated with point locations such as cities. Data is displayed on these maps with proportionally sized symbols to show differences in occurrences. Circles are most often used with these maps but squares and other geometric shapes are suitable as well. The most common way to size these symbols is to make their areas proportional to the values to be depicted with mapping or drawing software.

Another thematic map is the isarithmic or contour map and it uses isolines to depict continuous values like precipitation levels. These maps can also display three-dimensional values like elevation on topographic maps. Generally data for isarithmic maps is gathered via measureable points (e.g. - weather stations) or is collected by area (e.g. - tons of corn per acre by county). Isarithmic maps also follow the basic rule that there is a high and low side in relation to the isoline. For example in elevation if the isoline is 500 feet (152 m) then one side must be higher than 500 feet and one side must be lower.

A dot map is another type of thematic map and uses dots to show the presence of a theme and display a spatial pattern. On these maps, a dot can represent one unit or several, depending on what is being depicted with the map.

Finally, dasymetric mapping is the last type of thematic map. This map is a complex variation of the choropleth map and works by using statistics and extra information to combine areas with similar values instead of using the administrative boundaries common in a simple choropleth map.